Resin materials endowed with conductivity are used in a very wide range of applications such as anti-electrostatic products, surface-exothermic materials, CV cables, electrically conductive paints and ink, IC packing materials, shielding materials for electromagnetic waves and the like. There are also used resin tubes endowed with conductivity for the purpose of transporting fuel so as to remove static electricity generated by friction during the transportation and induced by charged gasoline through the filter.
As a means to impart conductivity to the resin, a method of adding to a resin, for example, carbonaceous materials like carbon black as a typical example is the most commonly conducted [JP-A-H07-286103 (1995) and JP-B-H08-13902 (1996), wherein the term "JP-A" as used herein means a laid-open publication for an unexamined application while the term "JP-B" means a publication after examined.] and, as for polyamide resin, it is well-known that an electrically conductive polyamide resin can be obtained by the mixing with conductive carbon black. There is a defect, however, that the heat-aging resistance of a polyamide resin is reduced when mixed with the conductive carbon black. Moreover, when such a resin is used as a tube for fuel oil, the oxygenated-gasoline resistance is far below the satisfactory level.
The addition of a phenolic or organophosphate heat-resistant stabilizer to a polyamide resin in order to improve heat-aging resistance is commonly known in the art but the effect is little in a polyamide resin mixed with the conductive carbon black and scarcely serves to an improvement.
JP-B-H03-61701 (1991) disclosed a method for improving heat-aging resistance by mixing polyamide containing conductive carbon black with copper iodide and potassium iodide. However, since copper iodide and potassium iodide are not compatible with a polyamide resin, it is difficult to make them uniformly dispersed in the resin. They also give rise to a problem that the surface condition of a tube obtained by extrusion (i.e. skin of the hose manufactured by extrusion) is deteriorated. Additionally, the oxygenated-gasoline resistance has been little improved.
There is also a report on the study of the effect of preventing oxidation by adding a phenol, secondary amine, hindered amine antioxidant to a monomer of nylon 6 (6-hexanelactam) [Lanska, B., Poly. Degrad. Stab., 53 (1996) 99-109] but this method does not relate to the addition of carbon black and the report does not investigate the effect of the antioxidant with regard to preventing aging deterioration of physical properties of nylon.